Radar (including sonar and lidar) has been known and used by man for 60 years or more. During this time, many improvements have been devised for correcting various sensor deficiencies and frailties. Among the early improvements was the suppression of clutter by adoption of moving target indication (MTI), which tended to suppress the display of targets which had little or no motion, as measured by lack of a pulse-to-pulse phase shift in the reflected signal echo. The problem of the inverse-fourth-power amplitude of the signal returns was substantially solved by increasing the gain of the radar receiving systems as a function of the fourth-power of time following transmission of each energy pulse, in order to maintain relatively constant amplitude of the target signal returns during processing.
Among the remaining problems of radar-type sensors are lack of good remedies for mutual interference between or among multiple radar sensors located in close proximity to each other. It is easy to understand that if two or more radars that are in close proximity to each other, transmit pulses in a common operating frequency band, each radar system will receive signals directly from the other radar(s) that are more-or-less indistinguishable from reflections from radar targets. These directly-received pulsed interference signals can be of large-amplitude and can tend to saturate receiving systems and target displays. This well-known problem manifests itself in increases in false alarm rates and in undesirable losses of sensitivity for detecting radar targets. Mutual interference is a serious problem for many types of radar, and is expected to be a continuing problem for future radar concepts.
More recently, pulse-Doppler radars were devised to improve upon MTI radars. Pulse-Doppler radars can operate in conditions where signals from severe clutter environments are received together with signals from intended targets. Pulse-Doppler radars use pulse-Doppler filters to convert the time-domain reflected radar signals to the frequency domain in order to suppress the clutter and maintain target detection sensitivity. Clutter suppression is accomplished by rejection or attenuation of those components of the signal returns that show zero or small frequency offsets from the transmitted signals. Mutual interference can be a more severe problem for pulse-Doppler radars than for other radars due to the inherent nature of pulse-Doppler signals and pulse-Doppler filter processing. The increased severity is due to the fact that if mutual interference occurs anytime during the transmission and reception of a pulse-Doppler waveform, detection sensitivity can be degraded for the entire waveform. And since pulse-Doppler waveforms can consist of long sequences of high-energy pulses, not only can mutual interference be more likely to occur, but it can also render useless more significant amounts of expended radar resources consisting of high-energy pulses.
In a simple prior-art arrangement, radar pulse-Doppler signal returns consisting of target and clutter signals are applied to a pulse-Doppler filter to transform the input time-series signal returns, into output Doppler-frequency domain signal measurements. This allows the system to resolve and separate the clutter signals from the intended target signals based on differences in their Doppler frequencies. The clutter signals are then suppressed either by setting their Doppler-frequency components to zero or by excluding them from detection processing. If pulsed interference occurs in the radar pulse-Doppler signal returns, together with target signals and clutter signals, then the pulsed interference appears on the output of all of the pulse-Doppler filters because of its wide frequency distribution. It can therefore mask the existence of intended target signals, which in turn degrades radar target detection sensitivity.
Methods which have been used in the prior art to ameliorate the problem of mutual interference for pulse-Doppler radars include blanking of the portions of the signals that are contaminated with pulsed interference; “repair” by interpolation of the portions of the signals that are contaminated with pulsed interference; pulse-Doppler filters that are adapted to the pulsed interference, and coherent sidelobe signal cancellation. While blanking effectively removes pulsed interference, it is not compatible with pulse-Doppler filter suppression of high levels of clutter signals. When blanking is used during occurrences of combined pulsed interference and clutter signals, the portions of the clutter signals that overlap with the pulsed interference are also blanked. Radar target detection sensitivity is degraded because blanking of portions of the clutter signals degrades pulse-Doppler filter capabilities for suppressing the clutter signals. This is a serious shortcoming because combined pulsed interference and high levels of clutter signals are a common occurrence. “Repair” by interpolation improves upon blanking by replacing clutter signals that are contaminated by pulsed interference with interpolated signals that are compatible with pulse-Doppler filter suppression of high levels of clutter signals. “Repaired” signals are obtained by applying an interpolation algorithm to the received signals that are temporally adjacent to the contaminated signals. (This process is sometimes referred to as “linear prediction”.) Interpolation fails if any of the temporally adjacent signals are also contaminated by pulsed interference. This is a serious shortcoming because contamination of temporally adjacent signals commonly occurs. Pulse-Doppler filters that are adapted to the pulsed interference solve the problem by synthesizing and applying a new pulse-Doppler filter for each change in condition of the pulsed interference. While this method is compatible with pulse-Doppler filter suppression of high levels of clutter signals, and is also compatible with occurrences of temporally adjacent signals also being contaminated by pulsed interference, its implementation is generally considered to be too costly to be practical due to the large amount of computer computation that is required to synthesize new filters. Additionally, it is not compatible with existing and optimized pulse-Doppler filter architectures that are based on fast-Fourier transform processing algorithms. Coherent sidelobe signal cancellation is an effective solution to pulsed interference, but only if a sufficiently large number of coherent sidelobe signal cancellation channels are implemented in the radar, and the levels of the pulsed interference are sufficiently low so that the receiver does not exceed its linear dynamic region. When the linear dynamic region of the receiver response is exceeded, intermodulation and cross-modulation signal products are produced which are incompatible with coherent sidelobe signal cancellation. This is a serious shortcoming because pulsed interference will commonly exceed receiver linear dynamic regions when radars are in close proximity.
Improved andor alternative mutual interference suppression for pulse-Doppler radar, sonar, lidar, and equivalent, is desired.